Provide an optional parameter for sequencediagram environment serving as a caption

```
I want achieve a result similar to Figure 2 at 
http://www.ibm.com/developerworks/rational/library/3101.html. A caption in a 
sequence diagram is useful for referring to it from other sequence diagrams. It 
should appear as if the sequence diagram is inside a block.
```

Original issue reported on code.google.com by `anakreon...@gmail.com` on 17 Nov 2011 at 2:48
